Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescence element and an organic electroluminescence element (hereinafter, referred to as an organic EL element). An inorganic electroluminescence element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescence element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescence element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
As a development of an organic EL element toward a practical application, there was a disclosure by a research group of Princeton University concerning an organic EL device using phosphorescence luminescence from an excited triplet (for example, refer to Non-patent document 1). Since the, investigations of materials which exhibit phosphorescence at room temperature have been actively done (for example, refer to Patent document 2 and Non-patent document 2).
Furthermore, in the organic EL element using phosphorescence luminescence recently discovered, since theoretically about 4 times of the luminescence efficiency can be realized compared with the formerly known EL element using fluorescence luminescence, the development of the materials, the layer structures and electrodes for it has been made all over the world.
For example, many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied (for example, refer to Non-patent document 3).
Thus, although the organic EL element using phosphorescence luminescence has a very high potential, this element largely differs from the organic EL element using fluorescence luminescence. It has been important technical investigational work in order to improve power efficiency and lifetime of the element such as: the way of controlling the location of a luminescence center, in particular, the way how to carry out light emitting stably by performing recombination inside the light emitting layer.
In recent years, there have been well known the multilayer lamination type element which is provided with a positive hole transport layer (located in an anode side of a light emission layer) and an electron transport layer (located in a cathode side of a light emission layer), both of which are adjacent to the light emitting layer (for example, refer to patent documents 2).
Especially, when blue phosphorescence luminescence is utilized, since the blue phosphorescence luminescent material itself has a high T1 (exited triple), development of surrounding materials and control of the precise luminescence center are strongly requested.
In recent years, in the light emitting layer of the organic EL element using a phosphorescence luminescent material, there were disclosed a technology of using a dibenzothiophene derivative as a host material (for example, refer to Patent document 3) and a technology using a dibenzothiophene derivative and a dibenzofuran derivative as a hole injection material and/or a light emitting material (for example, refer to Patent document 2).
However, from the viewpoint of providing an organic EL element exhibiting high luminous efficiency and low driving voltage, and excellent in thermal stability and raw stock stability, and moreover having a long lifetime, it is still insufficient and it is required further solutions.
On the other hand, the expectation for the wet method (it is also called a wet process or a wet coating process) is large from the request to a large size production, a low-cost production, and high productivity. As compared with film forming in a vacuum process, since a film can be formed at low temperature, the wet process can reduce the damage of an under-laying organic layer, and this technology is largely expected to achieve improvement of light emitting efficiency and an element lifetime.
However, in the organic EL element using blue phosphorescence luminescence, in order to realize wet film forming, especially the host compound contained in the light emitting layer and the electron transport material laminated on the light emitting layer will give investigational work.
From the practical viewpoint, it was revealed that that it is still insufficient and the further improvement technique is indispensable in respect of the solubility in a solvent, solution stability, driving voltage with the host material and electron transport material which have been disclosed until now.
Patent document 1: U.S. Pat. No. 6,097,147
Patent document 2: Japanese Patent Application Publication (JP-A) No. 2005-112765
Patent document 3: JP-A No. 2007-126403
Non-patent document 1: A. Baldo et al., Nature, vol. 395, pp. 151-154 (1998)
Non-patent document 2: A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000)
Non-patent document 3: S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001)